I would have asked for a reroute on a different airline. If I remember their contact of carriage does allow for a reroute in situations where a class of service is no longer available. I' ve had no problems getting rerouted instead of a dowgrade.

You have to push for it. Most low-level airline employees do their jobs very well but sometimes you get a lazy one and in this case you need to elevate this to a manager.

Just send a polite letter asking for a refund of 75% of your one-way fare. AC should respond quickly.